1. Field Of The Invention
The present invention relates to the bleaching of wood pulp. More specifically, the present invention relates to the sensing of residual caustic concentration in a paper pulp bleaching system and the exercise of control over the flow of caustic into a pulp bleach plant flow system.
2. Description Of The Prior Art
Since the objective of caustic treatment of paper pulp flowing chlorination and washing is to neutralize any residual chlorine and hydrolyze chlorinated lignin radicals for wash removal, the prior art control practice over caustic addition to the pump flow stream has been to add caustic as a function of either the chlorine addition flow rate or of the chlorine residual at some point along the chlorine retention flow path.
If metered as a function of chlorine addition, the caustic flow control system represents a feed-forward control over the caustic flow stream wherein caustic is added to the pulp slurry flow stream in some direct proportion to the quantity of chlorine added to the same approximate flow segment within the slurry stream. This system however, fails to acknowledge variables that are characteristically unique to successive flow segments of the continuous slurry flow stream. Such variables may comprise species differences in the wood furnish and the degree of digestion as represented by the pulp "K" Number. Other variables in the degree of chlorine reaction with a given slurry flow segment may include retention time (flow rate), temperature, consistency and pH characteristics. Such variables result in percentage variations of chlorinated lignin respective to successive slurry flow segments. Consequently, the quality of caustic necessary to both neutralize residual chlorine and hydrolyze chlorinated lignin will vary as a more complex function of the chlorine quantity applied.
U.S. Patent Application B Ser. No. 300,004 describes a paper pulp bleaching plant wherein caustic is combined with the slurry flow stream as a function of the chlorine residual at some fixed point in the chlorination system. Although the pulp "K" Number is also a function of the caustic demand determination, the system is extremely complex in the total quantity of instrumentation and telemetering required for a quantitative conclusion.
Such cumbersome and ungainly control techniques over the continuous metering of caustic into the pulp slurry flow stream as described above have resulted from a lack of means to directly and reliably measure caustic effectiveness on the slurry. Although probe cells such as the oxidation reduction potential (ORP) device described by T. C. Burnett, Pulp and Paper Magazine of Canada, Vol, 71, No. 14, July 17, 1970, pg. 57-62 may be used to measure caustic residual within a limited concentration range, beyond this range the instrument reverses polarity and proceeds irradically. Usually, at less than 8 grams of residual caustic (as Na.sub.2 O) per 100 liters of solution (points) and less than 9.3 pH, conditions critical to a control range which spans from 100.00 points residual and 11.0 pH to 0.0 points residual and 7.0 pH, ORP cell signals become false and unreliable.
It is, therefore, an object of the present invention to teach a method and apparatus for reliably measuring the concentration of caustic residual in a pulp slurry flow stream.
A further object of the present invention is to teach a relatively simple caustic flow control system regulated by residual caustic concentration sensors.